The effect of conjugation with double bonds and (in cyclopropane) the effect of ring strain on the cleavage of carbon-carbon bonds during thermal rearrangements is discussed. In particular, the Cope rearrangement and the geometry of its transition state are described. The structurally degenerate Cope rearrangement of bicyclo[5. 1. 0]octa-2,5-diene is discussed, and this compound is used to illustrate the concept of “fluctional structure”, which finds its extreme expression in the hypothetical compound, tricyclo[3. 3. 2. 04,6]-deca-2,7,9-triene.